1. Field of the Invention
The invention relates to a method and apparatus for processing image data of a text document, and more particularly, for handling the transmission of compressed data, representing the image data of the text document, between a terminal unit and a host processor.
2. Prior Art
In image processing systems, an image of an original document is typically scanned by an image scanner that includes an image sensor unit having a plurality of sensor elements. The number of sensor elements/inch defines the resolution of the image scanner. Each sensor element defines one pel and generates an analog signal representing a pel of the image of the original document. These analog signals are converted to digital signals which are compressed by an image compression algorithm well known in the art, such as, for example, CCITT standard Modified Read. The compressed data representing the image is then transmitted to a memory in a host processor.
When the image is to be utilized, the stored compressed data is read out and decompressed. The decompressed data is supplied to an output device, such as a display unit or a printer, which will reproduce the image. The scanner and the output devices may have various resolutions, e.g., 120 pels/inch, 240 pels/inch. In the situation where the resolution of the original image is 240 pels/inch and the resolution of the display unit is 120 pels/inch, it is clear that the display unit does not require input data of 240 pels/inch resolution. In such a case, however, compressed iimage data of 240 pels/inch resolution is transmitted from the host processor to a terminal unit where it is decompressed by an image decompression algorithm, to reproduce the original image at 240 pels/inch resolution. The reproduced image must then be scaled to generate the display image at 120 pels/inch resolution. Thus, while the display unit only requires input date of 120 pels/inch resolution, nevertheless transmission and decompression of the high volume of data involved with 240 pels/inch resolution is carried out. To avoid this problem, i.e., the transmission and decompression of the high volume of data, a solution might be for the host system to prepare both compressed data of 240 pels/inch resolution and compressed data of 120 pels/inch resolution and to store them in the memory. This approach, however, while solving the transmission problem, raises the problem of requiring about 1.5 times memory space for the compressed data of 120 pels/inch and 240 pels/inch resolutions.
U.S. Pat. No. 4,682,869 to Itoh et al discloses an image processing system wherein an original image is converted to a low resolution image, i.e., base image, by a pel density conversion algorithm. Then, the same density image as the original image is predicted from the base image; the predicted image and the original image are compared to generate error compensation data; and both the base image and error compensation data are compressed and stored for a later image reproducing process. This process is similar to the invention disclosed herein in constructing a two layered data base, but the present invention is remarkably different in providing an algorithm for compression and decompression of the second layer.
U.S. Pat. No. 4,901,363 to K. Toyokawa discloses a system for compressing bi-level data wherein history pels which are adjacent pels to a current pel being processed and pels so separated from the adjacent pels as to adapt to a period of a dither matrix, are sampled and it is determined from the sampled pels whether the image has a dither dominant image pattern or a text-graphic dominant image pattern. The teaching in that . application, however, does not involve the construction of a two layered data base.
Arranging image data in a hierarchical or pyramidal manner is well known in the art and described, for example, in U.S. Pat. No. 4,222,076 to K. C. Knowlton and in an article by K. R. Sloan, Jr. and S. L. Tanimoto, "Progressive Refinement of Raster Images," in IEEE Transactions on Computers, Vol. C-28, No. 11, November 1979, pp. 871-874. However, the image data structures have been devised with the purpose of data retrieving, and the data contents of the respective layers are different from those of the invention. Further, the hierarchical systems have a disadvantage that larger storage space is required as compared with the present invention.
Glen G. Langdon, Jr. in "An Introduction to Arithmetic Coding," IBM Journal of Research and Development, Vol. 28, No. 2, March 1984, pp. 135-149 describes the basic concept of arithmetic coding for compressing original data and for decompressing or reproducing the original data from the compressed data. The present invention draws from and utilizes the principles and techniques of this type of coding in achieving its improved results.